DiffServ and MPLS Packets

One thing you might have noticed is that there are 6 DSCP bits and only 3 EXP bits. Because only eight possible EXP values and 64 possible DSCP values (21 of which are currently defined) exist, how do you offer DSCP services over MPLS?

On a frame-mode network (as opposed to a cell-mode network), you are stuck with the 3 EXP bits; you need to map multiple DSCP classes to these EXP bits. However, this has operationally not yet proven to be an issue in production networks, because hardly any QoS deployments offer services that can't be provisioned with the 3 MPLS EXP bits.